Precipitation of M23C6 and .SIGMA.-phase in .DELTA./.GAMMA. duplex stainless steels.

Abstract

Precipitation of M23C6 and σ-phase during isothermal aging of δ/γ duplex stainless steels has been studied mainly by means of transmission electron microscope coupled with energy dispersive X-ray analyser. M23C6 particles form at both γ/γ and γ/δ boundaries. The particles are crystallo-graphically related to either one of the γ grains or the γ grain corresponding to the γ/γ or the γ/δ boundary. The M23C6 particles nucleating at γ/δ interphase boundaries grow into the untransformed δ-ferrite as an M23C6/new γ eutectoid structure dragging the γ/δ boundaries. The habit plane is the {111}γ//{111}M23C6 which is most parallel to the edgewise growth direction. Since C atoms to form M23C6 are supplied from the γ behind the eutectoid structure, the reaction will finally stop after consumption of C atoms. Then σ-phase particles nucleate on the δ/new γ interphase boundary in Nenno orientation relationship with the new γ, and grow into the δ-ferrite as a σ/γ cellular structure with the (001)σ//(111)γ habit plane. In some cases, inpingements between σ and γ grains nucleating separately occur, and result in a tangled eutectoid structure without any specific orientation relationship. Since the partition of alloying elements between γ and δ-ferrite phases accelerates precipitations of both M23C6 and σ-phase, the solution treatment in the δ-ferrite single phase region largely retards these reactions.